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snippets/python/s/when.md

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+---
+title: Apply function when true
+type: snippet
+language: python
+tags: [function]
+cover: interior-10
+dateModified: 2020-11-09T23:56:11+02:00
+---
+
+Tests a value, `x`, against a testing function, conditionally applying a function.
+
+- Check if the value of `predicate()` is `True` for `x` and if so call `when_true()`, otherwise return `x`.
+
+```py
+def when(predicate, when_true):
+  return lambda x: when_true(x) if predicate(x) else x
+```
+
+```py
+double_even_numbers = when(lambda x: x % 2 == 0, lambda x : x * 2)
+double_even_numbers(2) # 4
+double_even_numbers(1) # 1
+```